|Publication number||US8040212 B2|
|Application number||US 12/507,751|
|Publication date||Oct 18, 2011|
|Filing date||Jul 22, 2009|
|Priority date||Jul 22, 2009|
|Also published as||US20110018669, WO2011011624A1|
|Publication number||12507751, 507751, US 8040212 B2, US 8040212B2, US-B2-8040212, US8040212 B2, US8040212B2|
|Original Assignee||Volterra Semiconductor Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (63), Non-Patent Citations (16), Referenced by (3), Classifications (22), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present document relates to the field of low profile inductor design for high-density printed circuit boards. In particular, the document relates to a low profile inductor suitable for use beneath processor heat sinks and in other areas where conventional inductors may interfere with other components.
Many high density printed circuit board assemblies (PCBs) are installed in tight housings, or have bulky components attached to them, such that component height in portions of the PCB must be limited. For example, in the area near a processor of a personal computer motherboard, component height must be limited to avoid mechanical interference with processor heat sinks. Similarly, high profile components on PCMCIA or Cardbus devices are undesirable because they may require the device to occupy two slots in a laptop computer's connector instead of a single slot; occupancy of multiple slots may limit further system expandability and may prevent use with machines having only a single slot available.
Voltage regulated down-converters for providing power to microprocessor integrated circuits of laptop and desktop personal computers are known. Such converters typically include one or more inductors.
In an embodiment, an inductor for assembly on a printed circuit board includes a core formed of a magnetic material and a first foil winding wound at least partially around or through at least a portion of the core. A first end of the first winding extends away from the core to form a first extended output tongue, and a second end of the first winding forms a solder tab. The solder tab and at least a portion of the first extended output tongue are formed at a same height relative to a bottom surface of the core for surface mount attachment to the printed circuit board. The first extended output tongue is configured and arranged to supplement or serve as a substitute for a first foil trace disposed on a surface of the printed circuit board.
In an embodiment, an inductor for assembly on a printed circuit board includes a core formed of a magnetic material, a first winding wound at least partially around or through at least a portion of the core, and a first ground return conductor attached to the core. The first winding and the first ground return conductor are configured and arranged such that inductance of the first ground return conductor is not significantly increased by presence of the core, while inductance of the first winding is significantly increased by presence of the core, relative to an otherwise identical inductor without the core.
In an embodiment, an inductor for assembly on a printed circuit board includes an elongated ground return conductor forming at least one solder tab at each end of the conductor. The inductor further includes at least two spacer elements disposed on the ground return conductor and an elongated foil winding forming at least one solder tab at each end of the winding. The winding is disposed on the spacer elements such that the spacer elements separate the ground return conductor from the winding to create a channel between the ground return conductor and the winding.
In an embodiment, a printed circuit board assembly has a drop-in inductor attached to a printed circuit board. The drop-in inductor includes a first foil winding wound through an opening in a magnetic core and a first ground return conductor attached to the core. The first foil winding and the first ground return conductor are configured and arranged such that inductance of the first ground return conductor is not significantly increased by presence of the core, while inductance of the first foil winding is significantly increased by presence of the core, relative to an otherwise identical inductor without the core. The first foil winding and the first ground return conductor have ends formed as solder tabs for attachment to the printed circuit board, and the tabs of the first foil winding and the first ground return conductor are formed at a same height relative to a bottom surface of the core. The tabs of the first foil winding and the tabs of the first ground return conductor are attached to foil of the same layer of the printed circuit board. The printed circuit board forms an aperture, and the core of the inductor extends into the aperture.
In an embodiment, a printed circuit board assembly includes a printed circuit board, at least one switching device attached to the printed circuit board, and an inductor attached to the printed circuit board. The inductor includes a core formed of a magnetic material and a foil winding wound at least partially around or through at least a portion of the core. A first end of the winding extends away from the core to form an extended input tongue. At least a portion of the extended input tongue is soldered to and supplements a first foil trace disposed on an outer surface of the printed circuit board, where the first foil trace electrically couples the at least one switching device to the first end of the winding.
In an embodiment, a printed circuit board assembly includes a printed circuit board, at least one switching device attached to the printed circuit board, and an inductor attached to the printed circuit board. The inductor includes a core formed of a magnetic material and a foil winding wound at least partially around or through at least a portion of the core. A first end of the winding is electrically coupled to the at least one switching device, and a second end of the winding extends away from the core to form an extended output tongue. At least a portion of the extended output tongue is soldered to and supplements a first foil trace disposed on an outer surface of the printed circuit board.
It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. Specific instances of an item may be referred to by use of a numeral in parentheses (e.g., winding 1802(1)) while numerals without parentheses refer to any such item (e.g., windings 1802).
In a high density printed circuit board assembly, such as a processor motherboard 100 assembly (
In some systems, heat sink and fan assembly 106 may actually occupy only some of the space shown; however a system manufacturer may have reserved a larger volume to allow air to flow into the heat sink, and to allow for future use of a different heat sink or fan with future, faster, and even more power-hungry, processors. In other systems and subsystems, such as PCMCIA or CARDBUS cards, height restrictions may derive from other factors such as overall card or system dimensions. Further, component height is strictly limited in laptop systems because of desires to limit machine thickness.
Processor 103 draws considerable current since much of the power it consumes is at a low “core” voltage, typically between one and two volts, although voltage at the processor's “periphery” may be higher. The “core” voltage is typically provided by an on-board DC-to-DC down-converter. The DC-to-DC converter has one or more inductors, such as inductor 110, as well as several capacitors 112. Inductor 110 often has height 114 that would interfere with heat sink and fan assembly 106 if inductor 110 were located under heat sink and fan assembly 106. Inductor 106 is therefore located some distance away from processor socket 104. Similar situations may also arise with high performance graphics chips as these also consume considerable power and often require heat sinks.
A schematic diagram (
Output terminals of the phase inductors 206 are coupled together and to capacitors 208 and processor 210 via an output node (Vo) 218. If the connection from phase inductors 206 to capacitor 208 and processor 210 is made only via a typical thin foil PCB trace (e.g., trace 116
The low processor voltage, typically between one and two volts, and high processor current, often reaching peak currents of fifty to one hundred amperes, make the system quite sensitive to what may seem quite low parasitic impedances 212. For example, a current of one hundred amperes in a two-milliohm parasitic impedance is sufficient to provide a two hundred-millivolt drop; at a one volt core voltage, this may represent twenty percent of operating voltage. Such voltage drop due to the hundred amperes also relates to twenty watts of conduction loss and is environmentally undesirable, as this conduction loss represents power not used in the circuit, but is power used to produce heat wasted in the board layout.
It is desirable to minimize impedances 212, since these may not only waste power, but may allow processor 210 voltage to deviate outside desirable operating limits. The same arguments apply to parasitic impedances 214 between inductors 206 and power semiconductors 204, it is desirable to minimize these impedances also.
To minimize parasitic resistances in inductors 206, these inductors are often wound with one or just a few turns of thick foil (i.e., a conductive material such as copper having at least a substantially rectangular cross-section) or wire around or inside a powdered iron, ferrite, or similar ferromagnetic core suitable for use at the high frequencies—in the range 20 kHz to above 1 MHz—at which switching devices 204 typically operate. Multiple inductors 206 are often used, their outputs being connected in parallel and operated as a multiphase converter, to handle the requisite current. The foil with which inductors 206 are wound is typically significantly thicker than foil used for traces 116 on the PCB. In the embodiment of
Inductor 300 further includes an extended output tongue 308 extending away from core 306. Extended output tongue 308 has a thickness similar to that of winding 304, and extended output tongue 308 is electrically coupled to one end of winding 304. Extended output tongue 308 is, for example, an extension of winding 304—such configuration may help simplify construction of inductor 300 and/or reduce combined resistance of winding 304 and extended output tongue 308. At least a portion of extended output tongue 308 is configured for attaching (e.g., soldering) to a foil PCB trace or solder pad. Although extended output tongue 308 is shown as having a width 402 which is the same as a width 404 of the portion of winding 304 that passes through or at least partially around core 306, widths 402 and 404 may differ. For example, width 402 may be greater than width 404 to help minimize impedance of extended output tongue 308. In motherboard applications, extended output tongue 308 is typically electrically coupled to an output node (e.g., a buck converter output node). However, inductor 300 is not limited to such uses. For example, extended output tongue 308 could couple to a power supply intermediate node.
Inductor 300 further includes a solder tab 310 electrically coupled to the other end of winding 304, for soldering to a foil PCB solder pad. In motherboard applications, solder tab 310 is typically coupled to an input node (e.g., a switching node in a buck converter). In alternative embodiments, solder tab 310 could alternately be replaced by a different type of connector, such as a through-hole pin.
At least a portion of extended output tongue 308 and solder tab 310 are, for example, formed at the same height relative to a bottom surface 316 of core 306 to facilitate surface mount connection of inductor 300 to a PCB. Some of such embodiments are capable of being placed on a PCB using pick-and-place equipment and soldered to traces or solder pads of the PCB using reflow soldering techniques (e.g., infrared reflow, hot gas convection, vapor phase reflow) or wave soldering techniques.
In some embodiments, solder tab 310 is replaced with an extended input tongue. For example,
Extended output tongue 308 may be used to provide a low impedance electrical connection to inductor 300. For example, extended output tongue 308 may be configured and arranged for supplementing a foil PCB trace connected to inductor 300. In some embodiments, at least a portion of extended output tongue 308 is formed for soldering to and extending along a foil trace on a PCB outer surface, thereby serving as a conductor in parallel with the trace. Extended output tongue 308 typically has a thickness that is much greater than that of the PCB trace—accordingly, extended output tongue 308 typically has a much lower electrical and thermal impedance than the PCB trace. Extending extended output tongue 308 along a PCB trace to supplement the trace may significantly lower the trace's effective impedance, thereby reducing voltage drop and power loss in the trace, as well as improving the trace's heat sink ability. As another example, extended output tongue 308 may be used in place of a PCB trace to provide a low impedance electrical connection to one end of winding 304, and thereby free up a PCB layer for other uses, such as to route signal traces. Similarly, extended input tongue 502 (
Extended output tongue 308 may also serve as a heat sink, thereby helping to cool inductor 300 and a PCB that tongue 308 is attached to. Extended output tongue 308 also typically has a low profile, which may advantageously allow use of rework equipment, pick and place equipment, and/or test probes in the vicinity of tongue 308. Furthermore, because extended output tongue 308 is part of inductor 300, extended output tongue 308 may withstand pressure from hot air rework equipment without being blown off a PCB.
In typical embodiments, winding 304 and extended output tongue 308 are formed of copper foil, such as between three and five millimeters wide, and from two tenths to one half millimeter thick. It is desirable for width 402 of extended output tongue 308 to be at least 1 millimeter to promote low impedance of tongue 308. The foil winding material typically used for winding 304 and extended output tongue 308 is substantially thicker than typical PCB copper foils (e.g., trace 116,
As the extended tongues discussed above (e.g., extended output tongue 308 of
One or more ground return conductors can be attached to an inductor to improve ground return conductivity in the inductor's vicinity. The ground return conductors, for example, are configured and arranged such that their inductance is not significantly increased by presence of the inductor's core, while inductance of the inductor's winding (or windings) is significantly increased by presence of the inductor's core, relative to an otherwise identical inductor without the core. As an example, the ground return conductors may be configured and arranged such that the inductor's core does not form a magnetic path loop around the ground return conductors. In such embodiments, the ground return conductors are external to core, and the ground return conductors may have an inductance similar to that of a PCB ground plane extending under a standard surface mount inductor (without ground return conductors), where the ground plane is in close proximity to the standard surface mount inductor's core.
In many applications, current flows from switching devices through the inductor and to a load. Return current typically flows from the load, through PCB conductive layers under the inductor, and back to the switching devices. Accordingly, use of an inductor including ground return conductors may reduce ground return path impedance while maintaining the PCB's general current flow path.
Additionally, attaching a ground return conductor to an inductor allows both the inductor and the ground return conductor to be placed in a single step, thereby eliminating multiple placement operations required for placement of a discrete inductor and a discrete conductor. Furthermore, applying a foil conductor to a PCB may be difficult due to the foil's flexibility, but attaching a foil ground return conductor to an inductor increases the conductor's rigidity and may thereby facilitate the conductor's placement on a PCB.
In some embodiments, each end of ground return conductors 804, 806 and each end of winding 808 form respective solder tabs at a same height relative to a bottom surface 816 of core 810 to facilitate surface mount connection of inductor 800 to a PCB. Ground return conductors 804, 806, for example, have a thickness similar to that of winding 808 and are significantly thicker than foil typically forming a PCB ground return plane. Accordingly, ground return conductors 804, 806 may be used to supplement (or replace) a ground return conductor in a PCB (e.g., a PCB 802), and thereby significantly reduce the ground return impedance in the vicinity of inductor 800. Since ground-return conductors 804, 806 are attached to core 810, and thus to inductor 800, they are easier to handle than discrete conductors and may be positioned by pick-and-place equipment simultaneously with positioning inductor 800.
Accordingly, inductor 800 may be used to provide a low impedance, two-way path for current between DC-to-DC converter (e.g., buck converter) switching devices and a load, as shown in the examples of
The configuration and quantity of ground return conductors 804, 806 may be varied, and examples of some variations are discussed below. Additionally, although inductor 800 is discussed in the context of winding 808 carrying current to a load and ground return conductors 804, 806 carrying ground return current, inductor 800 could be used in other manners. For example, one or more of ground return conductors 804, 806 could be utilized to carry current, such as current from a memory-keep alive power supply (not shown) to the load, instead of ground return current. Furthermore, inductor 800 is not limited to use in DC-to-DC converter applications. For example, some embodiments of inductor 800 could be used in inverter applications.
A variation of inductor 800 is shown in
Some embodiments of inductors with an extended tongue (e.g., inductor 300,
At least portions of extended output tongues 1806 and extended input tongues 1808 are, for example, formed at a same height relative to a bottom surface of core 1804 to facilitate surface mount connection of inductor 1800 to a PCB. Each extended output tongue 1806, for example, supplements or replaces a PCB trace connecting inductor 1800 to a load (e.g., a processor). Each extended input tongue 1808, for example, supplements or replaces a PCB trace connecting inductor 1800 to DC-to-DC converter switching devices. Although
In some systems, each winding of a multiple winding inductor (e.g., inductor 1800) may be part of a separate phase of a multiphase DC-to-DC converter, such as discussed above with respect to
It should be noted that the quantity of windings as well as the quantity and configuration of ground return conductors may be varied. For example,
In other embodiments, low profile inductors as illustrated in
State of the art switching devices generally have a height of less than one millimeter when assembled on a PCB. Other commonly used surface mount components, such as ceramic capacitors, also have a similarly low height. Inductors, however, typically have a height of several millimeters so that their cores have a sufficiently large cross section to keep core losses to an acceptable level.
Accordingly, in height restricted applications, it may be desirable to use a “drop-in” inductor disposed in a PCB aperture. For example,
Inductor 3900 advantageously utilizes the height on both side of PCB 3904, as well as the thickness of PCB 3904. However, the aperture required for drop-in inductor 3900 reduces the path for return current through ground plane or interconnect layers of the PCB in the vicinity of the inductor, thereby increasing the return path impedance and associated losses. For example,
Furthermore, inductor 3900 is often fragile when installed in a PCB aperture. In particular, inductor 3900's solder tabs 3906 typically support inductor 3900's entire weight because inductor 3900's core 3908 typically does not contact PCB 3904. Accordingly, solder tabs 3906 are typically subject to significant mechanical stress, and may cause core 3908, which is typically formed of a relatively fragile magnetic material, to crack.
At least some of the problems discussed above can be reduced or eliminated with a drop-in inductor including one or more ground return conductors. For example,
Inductor 4200 includes a winding 4204 wound at least partially around or through at least a portion of a magnetic core 4206 (e.g., formed of a ferrite and/or powdered iron material). Winding 4204, for example, extends through a channel in core 4206.
Inductor 4200 can be used, for example, to provide a path for return current, as shown by arrows 4304 in
In contrast to prior art drop-in inductors, use of inductor 4200 does not necessarily increase return path impedance. Ground return conductors 4208, 4210 are often of similar thickness to that of winding 4204 and are frequently ten to fifty times thicker than typical PCB trace foil thickness. Use of drop-in inductor 4200 may therefore significantly decrease return path impedance, despite a PCB aperture being required for inductor 4200. Furthermore, inductance of inductor 4200 is less affected by PCB layout than prior art drop-in inductors because return current flows through inductor 4200.
Moreover, because inductor 4200 provides a path for return current, a number of inductors 4200 can be spaced close together without having to allow for space between inductors for a return current path, such as spacing 4106 required between prior art drop-in inductors 4100 of
Winding 4204 and ground return conductors 4208, 4210, for example, have respective solder tabs 4302 electrically coupled to their ends to facilitate surface mount connection of inductor 4200 to a PCB. Solder tabs 4302 are typically formed at the same height relative to a bottom surface 4212 of core 4206 to facilitate surface mount connection of inductor 4200 to a PCB. In some embodiments, solder tabs 4302 are extensions of winding 4204 or ground return conductors 4208, 4210, which may facilitate manufacturability of inductor 4200. For example, winding 4204 and its respective solder tabs 4302 may be formed of a single foil winding. Each of solder tabs 4302, for example, connect to PCB traces on a common PCB layer.
Inductor 4200 may be more mechanically robust than prior art drop-in inductors. For example, in embodiments where winding 4204 is a relatively rigid foil extending through a channel in core 4206, winding 4204 may provide significant mechanical support for inductor 4200. In contrast, the soft, multi-turn wire winding of prior art drop-in inductor 3900 typically provides little to no mechanical support for inductor 3900.
Additionally, ground return conductor 4208, 4210 may increase mechanical robustness of inductor 4200. For example, solder tabs 4302 coupled to ground return conductors 4208, 4210 may provide additional points to support inductor 4200 on a PCB, thereby reducing stress on inductor 4200's solder tabs and consequently reducing the likelihood of core 4206 cracking. For example, if each of winding 4204 and ground return conductors 4208, 4210 have respective solder tabs 4302 coupled to their ends, inductor 4200 may be supported on a PCB at six different places, as opposed to prior art inductor 3900, which is supported at only two places. Furthermore, ground return conductors 4208, 4210 may promote overall mechanical strength of inductor 4200.
Drop-in inductors with ground return conductors may have other configurations. For example,
Ground return conductors 4606, 4608 respectively include clamps 4612, 4614 which may allow for easier clamping of the ground return conductors to magnetic core 4604. Clamps 4612, 4614 may also increase robustness, physical attachment strength, and heat sinking ability of ground return conductors 4606, 4608.
The concept of adding ground return conductors to drop-in inductors can be extended to inductors including multiple, magnetically coupled windings. For example,
Inductor 5300 further includes a ground return current conductor in the form of a return current structure 5308 to provide a low impedance path for return current.
Although return current structure 5308 is disposed on the bottom side of inductor 5300 in
Although structure 5308 is shown as a ground current return conductor, it could be modified to carry additional signals. For example, an alternate embodiment of structure 5308 includes two electrically isolated electrical conductors, where one conductor serves as a ground return conductor, and the other conductor serves as a low current power supply conductor (e.g., a conductor for a keep-alive power supply).
Core 5804 is, for example, formed of pairs of corresponding magnetic elements 5814, 5816 and 5818, 5820, as shown in
As discussed above, use of prior art drop-in inductors typically results in problems including significantly increased return current path impedance, poor mechanical robustness, and the need to separate multiple instances of the prior art drop-in inductors. However, drop-in inductors with ground return conductors, such as some embodiments of the inductors discussed above, may reduce or eliminate one or more of these problems, as previously discussed. Accordingly, the addition of ground return conductors to drop-in inductors may allow for use of drop-in inductors in applications where prior art drop-in inductors would be impractical. Use of drop-in inductors instead of standard (non drop-in) surface mount inductors may offer a number of advantages, such the following: (1) reduced inductor height relative to the PCB surface; (2) increased inductor core size and cross section, which helps minimize core loss; (3) reduced PCB surface area required for the inductors; and/or (4) inductor height being closer to that of other power supply components, resulting in improved power supply volume utilization.
Adding one or more ground return conductors to a drop-in inductor may also significantly reduce or eliminate inductance dependence on layout and/or PCB aperture configuration. In particular, adding one or more ground return conductors to a drop-in inductor helps minimize length of the inductor's current loop in output inductor applications, where the current loop is defined by the path current takes when flowing through the inductor to a load, and from the load back by the inductor. Inductance is affected by the current loop's configuration, and increasing the current loop's size generally increases inductance. Accordingly, by minimizing current loop length through use of ground return conductors, current loop length may be significantly or completely unaffected by PCB layout and/or aperture configuration, thereby reducing or eliminating inductance dependence on such application characteristics. In contrast, in the prior art drop-in inductor of
It is anticipated that the foil windings and ground return conductors described herein are considerably thicker, and thereby offer considerably lower sheet resistivity, than the one-ounce copper foil used on many printed circuit boards. It is further anticipated that the foil windings and ground return conductors described herein are made from a highly conductive material comprising primarily copper. In alternative embodiments, the foil windings and ground return conductors are made from a non-cuprous metal such as aluminum or steel having a solderable low resistance coating of copper, and in may in turn be plated with tin or an alloy comprising tin for enhanced solderability.
Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US9263177||Mar 19, 2012||Feb 16, 2016||Volterra Semiconductor LLC||Pin inductors and associated systems and methods|
|US9281739||Aug 29, 2012||Mar 8, 2016||Volterra Semiconductor LLC||Bridge magnetic devices and associated systems and methods|
|US20150364999 *||Jun 15, 2015||Dec 17, 2015||Rohm Co., Ltd.||Semiconductor integrated circuit and power supply|
|U.S. Classification||336/192, 336/200, 336/221, 336/222, 336/83|
|International Classification||H01F27/28, H01F5/00, H01F17/04, H01F27/29, H01F27/02|
|Cooperative Classification||H05K2201/10659, H01F27/2847, Y02P70/611, H01F17/06, H01F27/292, H05K1/181, H05K2201/1003, H05K3/3426|
|European Classification||H01F27/28C, H01F17/06, H05K1/18B, H01F27/29B|
|Sep 3, 2009||AS||Assignment|
Owner name: VOLTERRA SEMICONDUCTOR CORPORATION, CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:IKRIANNIKOV, ALEXANDR;REEL/FRAME:023192/0753
Effective date: 20090811
|May 1, 2012||CC||Certificate of correction|
|Apr 20, 2015||FPAY||Fee payment|
Year of fee payment: 4